如何查找字段在列中一次出现或在列中多次出现但在一组日期之间仅出现一次的记录？

Question

I am trying to write a query that brings up the details (invoiceNumber, name, phone & email) from a table but only where the entries match these specific requirements:

Return the rows where the email address appears only once in the whole table from any date AND Return the rows where the email address multiple times WHERE the email has only been entered ONCE within the last 3 months.

I am fairly sure that this requires nested statements but I have no idea how to go about setting it up.

Any help would be greatly appreciated!

Answer 1

Approach this by thinking about the email addresses. You can determine the frequency of appearance by doing a group by:

      select emailaddress, count(*) as TotalCnt,
             sum(case when invoicedate >= sysdate - 90 then 1 else 0 end) as LastThreeMonths
      from t
      group by emailaddress;

This gives you the information you need. The following query joins this back to the original table to get the rows you are looking for:

select t.*
from t join
     (select emailaddress, count(*) as TotalCnt,
             sum(case when invoicedate >= sysdate - 90 then 1 else 0 end) as LastThreeMonths
      from t
      group by emailaddress
     ) e
     on t.emailaddress = e.emailaddress
where e.TotalCnt = 1 or e.LastThreeMonths = 1